Improve bundle translatability checks

Problem

Currently to check whether an bundle is enabled for translation we have first to call entity_translation_enabled() and only then entity_translation_enabled_bundle(). This is a cumbersome DX and often leads to missing bundle-level translatability checks.

Proposed solution

We should merge the two functions and specify the entity bundle as an optional second parameter of entity_translation_enabled(). This is a non-backward compatible API change but a quick check on drupalcode.org revealed that no one is using the $skip_handler parameter outside ET.